Syllabic amplitude companding has been used for decades in wire-line telephone systems to reduce noise and crosstalk. Their first application to wire lines in the U.S. was made in the early 1940's by the Bell System. See, for example, Carter, Jr., C. W., et al., "Application of Compandors to Telephone Circuits," AIEE Transactions, Vol. 65, 1946, pp. 1079-1086, hereinafter referred to as "Carter". These early discrete-component compandors typically employed copper oxide varistors as gain control elements and were not suited for mobile radio applications. Telephone systems provide an ideal environment for companding since the wire-line channel does not typically include amplitude limiting or frequency shaping.
As is known, a compander consists of two units, a compressor and an expandor. The compressor and the expandor complement each other. Therefore, if the compressor inserts amplification in the channel, the expandor inserts an equal attenuation. By allowing the system to transmit at a higher signal level, the noise is reduced.
The realization that companding may reduce the level of noise and co-channel interference between speech syllables prompted an interest in its application to cellular telephone systems. Subsequently, compandor integrated circuits (IC's) were developed that made the use of companding practical for use in cellular telephone systems. Recently companding has been introduced to non-cellular land mobile radio systems in the radio spectrum at 900 MHz.
See, for instance, U.S. patent application Ser. No. 90,982, filed on Aug. 28, 1987, entitled "FM Communication System with Improved Response to Rayleigh-Faded Companded Signals," by applicants Bruce C. Eastmond (one of the applicants of the present application) and Donald L. Linder, and assigned to Motorola, Inc., now U.S. Pat. No. 4,893,347, granted Jan. 9, 1990, hereinafter referred to as "prior application Ser. No. 90,982".
See, also, U.S. patent application Ser. No. 91,160, also filed on Aug. 28, 1987, entitled "FM Communications System with Improved Response to Rayleigh-Faded Received Signals," by applicants Bruce C. Eastmond and Elliott W. Lum (the identical applicants of the present application), and also assigned to Motorola, Inc., now U.S. Pat. No. 4,893,349, granted Jan. 9, 1990, hereinafter referred to as "prior application Ser. No. 91,160".
Another compander system of the prior art is disclosed in Ernst Schroder, U.S. Pat. No. 4,270,103, issued May 26, 1981, hereinafter referred to as "Schroder".
It is a well-known characteristic of some prior art companding systems that both the compressor and expandor must be present in order to prevent unnatural sounding speech. As a consequence, companding has not been applied to land mobile radio systems due to the large existing equipment base which has conventional speech processing.
FIG. 1 shows a first prior art repeater system which uses standard land mobile transit and receive speech processing. There is shown a first subscriber 101 transmitting to a repeater 103 via a first radio path 121. There is also shown the repeater 103 transmitting to a second subscriber 105 via a second radio path 123. FIG. 1A shows a second prior art land mobile radio system which uses standard transmit and receive speech processing. There is shown a first subscriber 101A transmitting to a repeater 103A via a first radio path 121A. There is also shown the repeater 103A transmitting to a second subscriber 105A via a second radio path 123A. The speech signal originating at the repeater receiver which is present at output 130 is connected to control point 135, and the speech signal originating at control point 135 is connected to repeater transmitter modulation input 132.
The amplitude response of the prior art transmit and receive speech processing measured at 1 KHz is shown in FIG. 3 by curve 301 and in FIG. 4 by curve 401. The end-to-end response of the prior art speech processing is shown in FIG. 5 by curve 501. This approach has existed unchanged for decades. The modulation limiter 107A is required to prevent the transmitter from producing deviations due to modulation in excess of a rated system deviation. See, for instance, EIA Standard RS-316-B, May 1979, page 13. Since the typical microphone 109A output signal is 8.5 dB above the clipping threshold, the modulation limiter 107A also acts to raise the peak-to-average ratio of the speech signal frequency deviation which will improve the effective communications range of the transmitter 111A. The use of a clipper to improve the peak-to-average ratio of the speech signal is characteristic of land mobile and military narrowband FM radio systems, and distinguishes these systems from commercial FM broadcast and radiotelephone systems in which the typical microphone output signal is substantially below the clipping threshold so that the dynamic range of the modulation is preserved without excessive distortion.
The choice of the clipping threshold involves a compromise between the amount of peak-to-average ratio improvement and the amount of distortion present in the received signal. The presence of speech signal distortion may affect the readability of the speech signal and also increases the level of spurious emissions, sometimes called "splatter", at adjacent channel frequencies. It is possible that the microphone signal may greatly exceed the typical level 309 in FIG. 3 if, for example, the person transmitting is in danger or excited for some reason. Situations of this kind occur frequently in police or public safety radio systems and often produce highly distorted, difficult to understand transmissions at critical moments. It is also possible that the microphone signal may be significantly less than the typical level 309 in FIG. 3 if, for example, the person transmitting is not close to the microphone or does not speak loudly.